Audio compressors and expanders are well known devices which are used to modify the dynamic range of an audio signal. An audio compressor or expander may be considered as having two parts: an electronically controlled variolosser or gain adjusting device and a control system with associated circuits which generate control signals which control the gain of the device as a prescribed function of the input (or output) signal.
Control of the dynamic range of an audio signal is important in several applications. Many times the dynamic range of a desired signal may exceed the processing capabilities of available audio circuitry. In many telephone systems, the audio signals are multiplexed, resulting in a poor signal-to-noise ratio. Therefore, it is desirable to compress the dynamic range of the audio signals prior to multiplexing and expanding the audio signals at the receiving end to allow a wider dynamic range signal to be passed through the telephone channel.
Another application for audio compressors are devices for the hearing impaired. In many cases, the response of the ear of a hearing impaired person will be substantially different, in terms of sensitivity and frequency response, than that of a normal person. Studies have shown that hearing aids with audio compression provide improved syllabic comprehension for persons with sensori-neural hearing losses. The use of audio compressors for the hearing impaired is described extensively in a report written by Walker and Dillon, entitled "Compression In Hearing Aids: An Analysis, A Review And Some Recommendations," NAL Report No. 90, published by the Australian Commonwealth Department of Health, National Acoustics Laboratories, June 1982.
Audio compressors may also be advantageously employed to tailor the characteristics of a hearing aid device to compensate for the deficiencies of individual users or to simulate normal hearing under a variety of situations such as very quiet or very noisy environments. For example, in many cases, a hearing impaired person will only experience a hearing loss at high frequencies and at high audio levels. Therefore, it may be desirable to alter the gain of the compressor as a function of the frequency and audio level of the audio signal. For example, in extremely noisy environments, it may be desirable to lower the high frequency gain of the compressor and in quiet environments, it may be desirable to increase the high frequency gain of the compressor. Audio compressors are readily adapted for both of these functions. These concepts may be further understood by referring to Villchur, E., "Signal processing to improve speech intelligibility in perceptive deafness," Journal of Acoustical Society of America, Vol. 53, pp.1646-1657.
For many types of hearing impairment, the degree of hearing loss is audio level and frequency dependent. In other words, the degree of relative hearing loss at high frequencies will vary with the audio sound level. Thus, the amount of correction that must be applied should also be varied with audio signal level. Usually, more high frequency correction is applied at low signal levels. Since each hearing impaired person has a unique hearing response, a compression system with adjustable compression ratio and frequency response is highly desirable.
Audio compressors are characterized in two categories. If the control signals are derived from the input signal of the compressor, the compressor is said to be of the feedforward type; if the control signals are derived from the output of the compressor, the compressor is said to be of the feedback type. The feedforward configuration requires that the dynamic range of the gain control circuitry be equal to that of the input signal. The advantage of this configuration is that the circuit is inherently stable. In a feedback configuration, a larger range of signals can be accurately processed since the gain control signal samples the output signal which has already been compressed. However, instability is often a problem with feedback compressors.
Many variations in the topology of compressors have appeared in the past. One example of a variable gain circuit for use with compressors or expanders is the Signetics NE570 which is primarily adapted for use with telephone systems. Another example of a prior compressor is disclosed in U.S. Pat. No. 4, 112,254, invented by Blackmer and entitled "Signal Compander System".
While the advantages of compression have been recognized, each of the above-mentioned systems has used compressors having a relatively fixed compression ratio or has been limited to uses where a relatively high voltage is available to power the compressor. The trend in the hearing aid industry today is to design hearing aid devices in increasingly small packages and some models today are tiny enough to be implanted in the ear canal. The advantages of compression have heretofore been unavailable in a canal aid device because these devices are typically operated from a single cell battery which provides a relatively low power supply voltage with limited current output. Prior to the present invention, a programmable compressor system having a continuously variable compression ratio, a programmable frequency response, and which is capable of operating from a 1.2 V battery has proved impossible to manufacture.